There have been many proposals in the past for frangible couplings for supporting highway sign posts and the like. Such frangile couplings are usually provided with one or more horizontal grooves defining a general plane of weakness, such as shown in U.S. Pat. Nos. 3,820,906 and 4,071,970. In constructing and installing such couplings, a trade-off is provided between the ability of the coupling to "give" when subjected to a horizontal impact and the ability of the coupling to support heavy uprights, signs, and the like, especially when the upright is subjected to wind loading. This trade-off exists because both a horizontal impact loading which desirably causes failure along the horizontal groove, and the tensile and wind loading of the upright act in the same way on the coupling. Thus, the coupling grooves must be made deep enough so that the coupling will fail when subjected to a horizontal impact, yet not so deep that the coupling will not support the weight of the upright, especially under wind loading.
According to the present invention, coupling means are provided that will fail easily in an out-of-plane tearing mode when subjected to a horizontal impact, yet is very strong with respect to weight and wind loading. According to the present invention, under impact loading, the coupling means is loaded in bending, for which there is a high stress concentration because of the geometry of the coupling means, and so that under weight or wind loading the coupling means is loaded in shear, where the stress concentration is low or zero.
Segregation of the different types of loading is accomplished according to the present invention by providing coupling webs comprising means defining at least one linear generally vertical groove therein, and means defining a through-extending opening adjacent at least one end of the groove and generally transverse thereto, the opening defining a first portion of the web on one side thereof and a second portion of the web on the other side thereof. Means are provided for attaching the first web portion to a generally vertical upright member, and means are provided for attaching the second web portion to a generally vertical base member. When one or more such webs are used to connect an upright and base together, upon subjection of the upright to a horizontal impact loading the web tears along the groove, high stress concentration being provided. However, when the upright and sign provide weight or wind loadings to the coupling, shearing forces are exerted on the groove, there being no stress concentration for this condition and thus the coupling being able to sustain a load limited only by the metal shear strength along the section at the groove times two, being able to support much higher wind and weight loadings than impact loads. Load ratios of 50:1 or greater are easily achievable with this system.
The coupling means according to the present invention also are fairly insensitive to direction of impact, providing an operational zone of impact of at least about 40 degrees, 20 degrees on either side of the groove, and some embodiments begin essentially directionally insensitive. A wide variety of geometrical configurations may be provided as the coupling means according to the invention, some geometries being less direction-sensitive than others, but all such geometries being capable of supporting high weight and wind loadings but yet fracturing easily under horizontal impact loads.
In order to facilitate manufacture of the couplings according to the invention, they are formed having a plurality of through-extending slits communicating with a through-extending opening in a web providing the coupling. The ends of the slits terminate short of the ends of the web, and have out-of-plane loading stress concentrating terminations. The provision of the through-going slits makes the couplings easier to manufacture, and manufacturable by a wide variety of techniques. Utilizing such through-going slits, the couplings can be manufactured more easily by casting, and may readily be stamped, a method of stamping the coupling, especially from steel, also being provided according to the invention.
It is the primary object of the present invention to provide readily manufacturable coupling members for uprights that will break away when subjected to a horizontal impact. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.